1. Field of the Invention
The present invention relates to analog amplifier circuits, and in particular, to analog amplifier circuits for receiving AC-coupled input signals and producing DC-coupled output signals.
2. Description of the Related Art
Referring to FIG. 1, analog amplifiers, such as audio power amplifiers, are often required to operate from a single power supply. This means the AC input signal (VIN) must be AC-coupled to the input of the amplifier circuit since the internal AC reference, or "ground," for the amplifier circuit is at DC voltage between DC circuit ground and the power supply potential, e.g. at a value equal to one-half of the applied DC power supply voltage. This AC reference is established by a resistive voltage divider (R.sub.1 and R.sub.2) with a bypass capacitor (C.sub.B) for charging to and maintaining the DC reference voltage across the lower resister. (In this particular circuit, as is often the case for audio power amplifiers, two serially cascaded inverting amplifiers are used in a bridge-tied-load configuration to drive a load, e.g. a speaker, with a differential output signal.)
However, this type of circuit suffers from a problem due to the necessity of having an AC-coupled input. Upon application of DC power (V+) to this circuit, the bypass capacitor begins to charge, as does the input coupling capacitor (C.sub.C) which is grounded at the input side by the output impedance of the grounded input signal source. Since the RC time constant for the bypass capacitor and its associated charging resistance are typically different from that for the input coupling capacitor and its associated charging resistance the capacitors charge at different rates. This results in the two capacitors having, at any given points in time during their charging or discharging periods, different voltages across them. In turn, this causes a transient signal to appear across the load. For example, during initial circuit turn-on, the current for charging the input coupling capacitor flows form the output of the first amplifier through its feedback (RF.sub.F1) and input (R.sub.I1) resistors. The resulting signal at the output of the first amplifier appears at the load in the "negative" portion (VOUT-) of the differential output signal, with the "positive" portion (VOUT+) applied by the second, cascaded inverting amplifier. This initial signal across the load is a turn-on transient which in the case of an audio power amplifier produces a "click" or "pop" from the speaker. (Similarly, during circuit turn-off, a turn-off transient produced by unequal discharging of the capacitors will often produce a "click" or "pop" from the speaker as well.)
Similar turn-on and turn-off transients occur in single-ended circuits, i.e. those amplifier circuits in which a single-ended output signal is provided to a grounded load (e.g. either VOUT- or VOUT+ only) rather than a differential output signal to a load isolated from circuit ground.
Conventional amplifier circuits have been developed which address this "pop" problem in a number of different ways. One approach has been to avoid using single power supply circuits by biasing the amplifier circuit between equal positive and negative power supply voltages with the output driving a grounded load. This allows the input coupling capacitor to be eliminated, thereby eliminating the cause of the turn-on and turn-off transients. However, this requires a second power supply which increases system complexity and costs. Another approach has been to apply the single DC power supply voltage in a gradual manner to initiate the flow of DC bias currents within the amplifiers. However, this results in the amplifier circuit having an indeterminate state of operation during turn-on and turn-off. Further, turn-on and turn-off transients can still occur when power is removed and quickly reinstated as in when a system reset is performed.
Accordingly, it would be desirable to have an analog amplifier which can be operated with a single power supply and reduced turn-on and turn-off transients.